Pros and cons of the various methods of joinery.
There are two types of forces that impact a shutter’s integrity. These are called racking forces and shear forces. Simply put, racking forces are caused by gravity pulling the panel down while the hinges are holding it up. Shear forces are caused when a panel is opened and closed during normal use and exacerbated when a panel is slammed shut or yanked open. This is all expected use, and a quality shutter will hold up to these conditions where a poor quality shutter will fail. Let’s look at five commonly used methods of joining shutter stiles and rails.
Biscuits are commonly used for small panels like kitchen and bathroom cabinet doors. They don’t provide much strength but are used to help align the stile and rail for a clean corner while the glue sets. Biscuits are never used in fine shutter joinery as they lack the strength to support large panels. Thankfully, most shutter companies have moved away from biscuits for shutters.
Bridle joints are good for withstanding sheer forces, so they do have a place in the production of fine furniture. But shutter panels have to be able to withstand racking forces too, and the bridle joint is not suitable for this purpose. Some well-known manufacturers use a bridle joint, but they call it an “open mortise and tenon.” There is no such thing outside the imagination of a marketing agent.
Dovetail joints are good at withstanding push and pull forces, but like the bridle joint they are inferior when subjected to racking forces. Unlike the bridle joint they perform poorly when subjected to shear forces. While a great joint for drawers, the dovetail is a gimmick when used for shutters. Beware of shutters built to the standards of the marketing department rather than the engineering department.
Dowels are the best joint so far on this list, and are used by many top manufacturers. Many of the nicest and most costly shutters on the market use large dowels to secure their rails to stiles. The limitation is the adhesive surface area is equal to or less than the surface area of the dowel. Fluted or spiral dowels are superior to smooth dowels as the flute allows for more adhesive to be used. Dowels are fine for withstanding racking forces, but have a tendency to fail when exposed to shear forces. Thankfully this is a repairable issue and a total replacement can be avoided.
Mortise and Tenon is the best joint for a shutter panel, bar none. The tenon is formed from the same piece of wood as the rail, and a deep mortise in the stile allows for maximum contact area for the wood adhesive. Panels built with mortise and tenon are sturdier and longer lasting than those built with any other form of joinery. I’m not sure we’ve ever gotten a call that a mortise and tenon were failing, but if they did, it would be an easy repair.
By Shem Isaac, Home Living Windows